Kate, wearing a blue LK Bennett dress, called the scene “amazing” and at one point gently wiped her eye.

The royals were guided to the spot to place their tributes by former head of the British Army Sir Richard Dannatt.

The poppies, which will eventually circle the Tower, are planted by volunteers – the last to be placed on Armistice Day.

They are available at £25 each, with proceeds going to military charities, whose representatives met the royals yesterday.

Mr Cummins said: “It is a way of showing the sheer number of those who lost their lives.”

There have been a string of high-profile events to mark 100 years since the start of the war.

So far 120,000 ceramic flowers have been planted but by the time its finished in November there will be an enormous 888,246.

Each one is hand made and unique, representing the number of British and Commonwealth servicemen and women killed during World War I.

The Duchess was joined on the emotional visit with Princes William and Harry, with all three planting their own flowers to join the huge red sea.

William was heard telling the artwork's creator Paul Cummins the piece was "spectacular" before they joined him to climb the Middle Tower to view the artwork from on high.

They then walked through the poppies before planting their own tribute to the war dead, accompanied by General Lord Dannatt, Constable of the Tower of London, and his wife Philippa.

Powerful Moment: Kate Middleton planted at her own poppy at the Tower, joining 120,000 others already in place

The smartly-suited Princes and Kate, who wore a dress of cobalt blue, also met volunteers from Historic Royal Palaces and ex-services personnel who helped install the artwork.

They then briefly walked around the moat to see the extent of the work so far while crowds lining the top of the moat cheered.

Mr Cummins said he got the idea for the artwork from a "living will" he found in Derbyshire two years ago which had used the phrase that gave his installation its name.

"Each one represents someone who died in the First World War from Britain and the Dominions. I'm literally trying to represent people because a number is a number, but if you see it all like this it is a visual idea of how many people were there."

Stage designer Tom Piper, who helped Mr Cummins make his vision a reality, said: "We also wanted to make sure it didn't become regimented mass rows of poppies - there is an organic quality to it - so taking the line of the 'seas of red', a feeling of waves of movement so that some of the poppies are higher than others so you can see them gently moving in the breeze... just to give it a sort of energy, the energy of all those who lost their lives."

The royal visit to the exhibition comes a day after they joined political leaders and relatives of the fallen in Belgium to remember the sacrifices and losses exactly a century on from Britain's entry into the war.

The Tower's part in the war started when more than 1,600 men swore an oath to the crown there in August 1914 after enlisting for war service.

Adam Gerrard / Daily Mirror

Spectacular Scene: Prince William was overheard to describe the art installation as 'spectacular during his visit this morning

The poppies in the moat take three days to make each and are for sale online at £25 per flower.

After Remembrance ceremonies in November they will be posted to the buyers, with profits divided between six service charities including Help for Heroes and the Royal British Legion. Each of the poppies is being installed by the artists and a team of 8,000 volunteers.

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Valuable Help: Around 8,000 volunteers have been installing porcelain poppies at the Tower artwork, which won't be finished until November

One of those volunteers is Joan Clayton-Jones, 72, from Sussex, who has made repeated trips to the capital to take part over the last few months despite the hard work involved.

She said: "When I saw it advertised I just had to come here, my great uncle was killed in the First World War and my husband's grandfather.

"It is my own personal act of remembrance, I just came because I wanted to do it. I have a photograph of my great uncle in my bag today."